In existing radio communication systems, such as a telecommunication system or a mobile cellular system, transmission parameters such as modulation, channel coding, precoding weights, and transmission rank (number of MIMO layers) are often dynamically determined in order to adapt the system to the varying channel conditions. In this manner, data rates in the systems, such as wireless networks, may be improved. An estimation of the channel characteristics is required to be able to determine such transmission parameters dynamically. The estimation of the channel characteristics is often performed at the receiver side of the channel whose characteristics is to be estimated. However, such estimation may also be performed at the transmitter side.
In the LTE standard, a mobile station, also known as a User Equipment or UE for short, estimates the channel characteristics, and selects an appropriate set of transmission parameters, such as transmission rank and precoding weights. The mobile station also estimates what channel quality may be obtained with the selected transmission parameters. The channel quality and the selected set of transmission parameters are reported to the first radio network node 110 in a channel quality report. Then the base station decides what transmission parameters shall be used. The base station may, hence, modify the transmission parameters reported by the mobile station to the base station.
In WO/2009/100775, there is disclosed a method for link quality estimation, in which the transmitter gets a channel quality report from the receiver. The transmitter also calculates an estimate of the channel quality. If the estimate differs from the estimate derived from the channel quality report, the transmitter sends an offset to the receiver, which uses the offset to adjust the estimate to be reported. An example method for the base station's quality estimate is to monitor the error rate using ACK/NACK reports from the mobile station. Such an outer loop is typically slow in compensating for too pessimistic values, since it only adjusts the channel quality estimate by a small amount (at most) at every reception of an ACK/NACK feedback from the mobile. Even for continuous traffic, the channel quality estimate in the base station will be pessimistic during a long time.